Known in the art is a helical type intake port comprised of a helical part formed about the axis of the intake valve and an intake air inflow passage part extending tangentially from the helical part, the intake air inflow passage part having a first side wall surface tangentially connected to the peripheral wall surface of the helical part and a second side wall surface extending toward the stem of the intake valve up to the peripheral wall surface of the helical part, wherein the top wall surface of the intake air inflow passage part is comprised of a first top wall surface positioned at the above-mentioned first side wall surface side and smoothly connected to the top wall surface of the helical part and a second top wall surface positioned at the above-mentioned second side wall surface side and lower in height than the first top wall surface, a lower layer flow flowing along the bottom wall surface of the intake air inflow passage part and an upper layer flow flowing along the first top wall surface are generated at a height position of this second top wall surface as a boundary, and this upper layer flow generates a swirl in the combustion chamber (see Japanese Utility Model Publication (A) No. 2-147830).
In this helical type intake port, when the intake air amount is large, the whirling action of the upper layer flow in the helical part is weakened by the lower layer flow and thereby generation of excessive swirl in the engine high operation region is prevented.
However, in this helical type intake port, in the same way as a conventional helical type intake port, a swirl is generated in the combustion chamber by making intake air to swirl in the helical part. In this case, to strengthen the swirl, it is necessary to strengthen the whirling action in the helical part. However, if strengthening the whirling action in the helical part, the intake resistance increases, so the charging efficiency falls and as a result the output at the time of maximum load operation falls.
In this way, so long as strengthening the whirling action in the helical part to strengthen the swirl, it is difficult to simultaneously secure a powerful swirl and high charging efficiency. To simultaneously secure a powerful swirl and high charging efficiency, a complete switch in thinking is necessary.
The inventors researched the method of flow of intake air for a long time up to this and finally discovered an intake port enabling a powerful swirl and high charging efficiency to be simultaneously secured.